1. Technical Field
The present invention relates generally to memory devices, and more particularly to circuitry that reduces or eliminates data dependent power supply noise when sensing a memory cell.
2. Background Art
For most semiconductor memory devices, including non-volatile memories, a sense amplifier (sense amp) is used to detect or "sense" the state of a memory cell. The continual scaling of these memory devices results in an ever decreasing amount of current, charge or voltage variation that these sense amplifiers must detect to discern the memory cell state. When a current sense technique is used, the sense amplifier circuit will usually sink or source a different amount of current depending on the "state" of the memory cell being detected. Since typically multiple sense amplifier circuits share a common power supply and are implemented in parallel to concurrently read multiple memory cells, these currents are additive and can lead to a "data dependent" power supply variation or "spike" (i.e., current and/or voltage variation). For example, in the extreme case, if all the memory cells to be read in parallel were in the same state, then the collective current sunk or sourced by the power supply upon reading these cells can be rather significant, and may cause a voltage spike as the power supply cannot maintain instantaneous voltage regulation in response to the rapid and significant power (current) demand. Evidently, various combinations of the memory cell states source or sink different amounts of current, and may result in different characteristic voltage variations. These unwanted variations in electrical consumption (i.e., variation in supplied voltage and/or supplied/sunk current) represent noise that can make it difficult for the sense amps to detect the correct data.
Additionally, in systems and devices where such memory devices are used to store secret information (e.g., private keys) for security and privacy (e.g., in a smart card, a mobile telephone, etc.), these data dependent power supply variations may compromise security of the device and the system or network in which the device is used. More specifically, by monitoring the data dependent power supply variations (e.g., the current) while the secure device performs various operations involving secure data reads (e.g., establishing a communication link with another device, performing monetary transactions, etc.), sufficient information may be acquired for determining the "securely" stored information (e.g., private key).
U.S. Pat. No. 4,932,053 generally addresses this latter security issue associated with data dependent power supply variations by introducing a random current so as to "mask" the current changes and thereby the true value of any data that is being read from memory. Such random currents are introduced by using additional simulation memory cells and a psuedo-random generator for controlling them, thus requiring a significant amount of chip real estate, which real estate may be particularly limited and valuable for various types of secure devices (e.g., smart cards).
U.S. Pat. No. 4,916,333 also addresses this security problem, and discloses a binary logic level detector that has essentially the same electrical consumption regardless of the logic level detected. To provide this essentially data independent consumption, the binary logic level detector consists of two parallel-connected identical read amplifiers that take complementary logic states when they receive the same logic level to be detected, thus essentially doubling the chip real estate required for detecting memory cell logic levels, which is particularly not well suited when using high sensitivity sense amplifiers that employ numerous transistors.
It may be appreciated, therefore, that there remains a need for further advancements and improvements in reducing, eliminating, or minimizing power supply variations when sensing the state of a memory cell, and particularly for circuitry that provides reduced data dependent power supply variations without requiring significant chip real estate, while being well suited for implementation with conventional sense amplifier designs.